1) In the area tab, under boundary Checking.. Select Edge. Press CONTROL when selecting it to preview it is the view port. To be honest though, mulch, if you want it to look realistic, live gravel and even grass, tends to have a ragged edge, so this is never perfect, but I agree, the way patches are created, isn't the most efficient way
2) If the patches of mulch are too large for the area, try reducing the scale of the much . Geometry tab, adjust the SCALE % under the Z Offset, to reduce the scale of each patch or Global Scale to reduce the scale of all patches, you will of course have to adjust the distribution to compensate
3) Model the surface for the mulch, so that it's edges go below that of the surface surrounding it and / or maybe lower the mulch surface very slightly
Try a fall off map, to reduce the dsiribition and scale of the mulch toward the edges and use a soil map on the surface, along with a bit of the displace modifier, to prevent visibility of gaps showing in the render
Go for it and create your own, by editing the presets. Have never had this problem, so have never had to do this myself. I would use a soil map and displacement.
I have been thinking for a while, of creating patches using hexagons rather than circles, which the majority of those patches use, because the hexagon, is the most efficient shape (bees knew that long before us humans figured it out).... Sadly I never seem to have the time to create new patches for grass, gravel and mulch etc...
Who knows, maybe this will be one of the new features of the upcoming Forest Pack Pro 7... One thing that I find very annoying about Forest Pack Pro however, is the Forest Templates layer... My thinking is if Railcone doesn't need the original geometry in the scene, why does Forest Pack pro, why can it not reference the geometry, rather than have it in the scene, frozen and hidden...Anyway, that's a totally different subject.
Anyway hope the above helps, solve your problem